1. Field of the Invention
This invention relates to a holographic information storage apparatus, and more particularly to an apparatus which stores audio, video or computer data in holographic form upon a rotating disc.
2. Description of the Prior Art
The desire for an alternative to the familiar LP record has led to the invention of numerous information storage devices. These devices, using either analog or digital formats, attempt to alleviate the deficiencies inherent in the LP record, and provide a medium for the storage of video as well as audio information. Two serious shortcomings associated with the LP record are noise (caused by scratches, dust, wear, manufacturing imperfections and mechanical motion) and limitations of frequency response imposed by mechanical detection of the signal, the latter problem preventing the LP record from becoming a medium for the storage of video information.
Prior-art devices which seek to correct these deficiencies have spawned many new problems, not the least of which is the relatively high costs associated with the manufacturing of both the playback devices and copies of the recorded medium, these costs being attributable, in part, to the close mechanical tolerances necessary for proper operation. Devices which do not rely upon a tracking groove on the surface of the recorded medium frequently resort to a sensing element along with a servo-feedback system for keeping the data-track centered with respect to the signal detection element; this mechanism contributes significantly to manufacturing cost. Other devices, employing optical detection of a signal, attempt to solve the tracking-error problem by exploiting the spatially-invarient characteristics inherent in certain hologram-forming geometries (Fourier-transform holograms); carried to the extremes necessary for allowing abandonment of a servo-feedback system, this solution severely limits the data storage capacity of the medium.
Some devices are intended to offer application in the area of computer data storage (in the form of a read-only block-access memory system) in addition to serving as an audio and video playback mechanism. However, to accommodate all three of the above applications requires either extensive modification of the device or inclusion of one or more additional elements, both solutions contributing to large increases of overall cost. Of particular significance is the signal detection apparatus; unless the design of this apparatus permits very rapid access to separate blocks of data on the recorded medium the utility of any information storage system is seriously compromised when applied to computer data storage. A device which permits the simultaneous storage of audio, video and computer data upon the same recorded medium is advantageous especially where primary application for such a device will be found in the home; prior art devices do not offer such multiple applications at a cost which is acceptable within the consumer marketplace.
Devices which store information in the form of a spiral track or series of concentric tracks upon a rotating disc encounter a problem associated with nonuniform data-density storage. That is, a disc presents much longer tracks and therefore more potential data storage space at its perimeter than exists toward its center; moving at constant rotational speed a disc wastes the potential for greater data-density which exists at the outer portions of a spiral track. Prior-art devices attempt to solve this problem either by adjusting the speed of rotation of the disc, or by providing a separate timing signal during playback. Both solutions increase cost and complexity, but the present invention provides an inexpensive solution which is intrinsic in the concept of the device.
Holographic data storage devices offer certain advantages over other optical systems, among the most important being data redundancy and the capacity for copies of the recorded medium to be manufactured by a pressing or stamping process as is well known in the art. Data redundancy not only alleviates the problem of noise created by dust, wear and scratches upon copies of a master, but also contributes to the low cost of mass-producing these copies by eliminating the necessity for maintaining a totally sterile environment during the manufacturing process. The production of holographic copies could be accomplished by passing a continuous thin sheet of either transparent or reflective material between two rotating drums, one of which would contain a relief pattern upon its surface (the master) and the other drum would be either of smooth surface or a second master (in the case of a record containing data on both sides). The resulting embossed hologram copies would be covered on each side with transparent material for protection, stiffening and labeling purposes, then cut to appropriate dimensions. The production speed and high volume attainable by such a process would match that which is commonplace within the publishing industry, and the manufacturing cost per copy would be very low.
A further advantage of holographic systems is afforded by the capacity of holograms to be copied also by a photographic or contact-printing process, thereby permitting the inexpensive production of copies in a very limited number. This attribute is important within the marketplace, where consumer demand for certain audio-visual programs involving, for instance, the performing arts might be insufficient to warrant the expense of preparing for production of copies via the high-volume process described above. Thus, the diverse needs and tastes of the public can be better served by holographic information storage devices than by other optical systems.
Prior-art holographic devices which warrant comparison with the present invention are as follows:
Takeda et al, U.S. Pat. No. 4,021,606, describes a device which employs oblong Fourier-transform holograms for purpose of reducing motional sensitivity. It differs significantly from the present invention in that the signal images (as opposed to the holograms themselves) are elongated in a direction parallel to the movement of the data carrier. Also, neither the signal images nor the separate holograms overlap. Only audio information is stored in holographic form upon the rotating disc, video information being recorded adjacent to the audio track; neither is the device intended to serve as a computer-data storage device.
Satoh et al, U.S. Pat. No. 4,104,489, also relies upon Fourier-transform holograms. And, like the previous patent, the signal images occur in a specific plane relative to the hologram recording. Tracking of the signal image path is accomplished using a servo-feedback system in conjunction with a differential photodetection system, this apparatus being employed during lateral movement of the turntable (the signal detection system being stationary).
Four patents describe devices employing superimposed holograms (or superimposed diffraction gratings), a primary feature of the present invention. Lamberts et al, U.S. Pat. No. 3,392,400, describes a device employing superimposed sets of equally-spaced diffraction lines, each set representing one digit; the device does not use holographic techniques for creating these diffraction lines, nor does the device permit the superimposition of adjacent data tracks. Brooks, U.S. Pat. No. 3,812,496 describes a device which uses holographic techniques for recording superimposed data tracks on a moving strip of film, but does not employ coded reference beam multiplexing (as does the present invention); nor does the device permit the non-simultaneous recording of adjacent data tracks during multiple exposures of the same photosensitive surface of the recorded medium. Silverman, U.S. Pat. No. 3,753,249, describes a device which records "spot patterns" elongated in a direction parallel to the motion of the rotating data carrier, these patterns substantially overlap one another; the device does not use overlapping adjacent data tracks, nor does the signal detection apparatus resemble that which is employed in the present invention. Marko, U.S. Pat. No. 3,848,096, describes a device which sequentially superimposes an entire aggregate of data on top of the same photosensitive material; unlike the present invention no area of the photosensitive material is masked during the exposure process and, therefore, no limits are set on the number of exposures to which the photosensitive material is subjected; the data carrier remains stationary during recording and playback, and a servo-feedback system is employed to accurately follow the data track. This last invention does not describe a device which addresses the problem that diffraction efficiency of a hologram is reduced by the square of the number of superimposed exposures, as is well known in the art.
In addition to the previously mentioned Brooks and Satoh patents Kamisaka et al, U.S. Pat. No. 3,903,360, describes a device which employs a differential photodetection system for the purpose of separating a time-varying signal from a simultaneously recorded carrier frequency. Unlike the Brooks, Satoh and Kamisaka devices the present invention uses a differential photodetection system for the purpose of extracting noise from the data signal, this noise being the consequence of using coded reference beam multiplexing during the recording process.
Among prior-art devices employing a rotating disc as the data carrier Kiemle, U.S. Pat. No. 3,770,886, employs a needle to track a spirally inscribed groove on the disc. Of particular significance is the fact that off-axis holography is employed to record video information either at the base of the groove or adjacent to it. Unlike the present invention, however, the holograms employed in the Kiemle device are recorded in one dimension only, this dimension being in the direction of movement of the recorded medium. No overlap of adjacent tracks is employed, nor is there any attempt to record both audio and video information holographically, the former being recorded as undulations in the groove itself as with conventional LP records. Broussaud et al, U.S. Pat. No. 3,842,197, describes a flexible disc upon which have been recorded a "mosaic" of holograms in spiral form, and for track-guiding purposes a second set of holograms has been superimposed upon a first set during a two-stage recording process. Unlike the present invention the two separate sets of holograms are not multiplexed through a coded reference beam procedure, nor is there substantial overlapping of holograms in circumferential and radial directions (although the possibility of "slight overlapping" is referred to in the discussion of the preferred embodiments--see column 2, line 32). Rembault, U.S. Pat. No. 4,025,731, also employs a holographically-recorded disc, but this disc does not include holograms which overlap in both circumferential and radial directions, nor does the recording process employed to create such a disc include the nonsimultaneous multiplexing of signal images.